Charged particle neutralizing apparatus and method of neutralizing charged particles

ABSTRACT

A method for use in neutralizing a charged discharge includes providing a neutralizer housing having a longitudinal axis extending between an inlet and an outlet thereof. A charged discharge is introduced into the inlet of the neutralizer housing for flow parallel to the inlet of the longitudinal axis from the inlet to the outlet. An alternating electric field is created within the housing parallel to the longitudinal axis for directing bursts of negatively charged ions and positively charged ions alternately towards the inlet for use in neutralizing the charged discharge. An apparatus for carrying out this method is also described.

STATEMENT OF GOVERNMENT RIGHTS

The present invention was made with government support from the NationalScience Foundation under Grant No. CTS-9304152. The Government hascertain rights in this invention.

This is a division of application Ser. No. 09/034,433, filed Mar. 4,1998, now U.S. Pat. No 5,992,244 which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to the neutralization of chargedparticles. More particularly, the present invention pertains methods andapparatus for neutralizing charged particles using a flow of ions.

BACKGROUND OF THE INVENTION

Neutralization devices are currently available for use in neutralizingcharged particles for a variety of applications, such as, neutralizationof charged nanometer particles for use in the development of standards,the use of such neutralized particles for structured materials, the useof neutralized particles for biotechnology applications, etc. Forexample, a neutralization device is described in U.S. Pat. No. 5,247,842(Kaufman et al.), entitled, "Electrospray Apparatus For ProducingUniform Submicrometer Droplets," issued Sep. 28, 1993. Theneutralization device is used in combination with an electrospraydevice. In the electrospray device, electrically conductive liquid issupplied at a controlled rate to a capillary tube. A voltagedifferential between the capillary tube and a surrounding chamber wallcreates an electrostatic field that induces a surface charge in theliquid emerging from the tube. Electrostatic forces disperse the liquidinto a fine spray of charged droplets. To produce the spray, eachdroplet is charged to about 80-95% of the Rayleigh limit (at which pointelectrostatic repulsion overcomes surface tension). Such electrospraydevices are used in many applications due to their ability to generatesmall and uniform droplets.

The electrically conductive liquid being sprayed is generally a liquidhaving particles dispersed therein. The particles, e.g., particles of asuspension, and the liquid is sprayed using the electrospray device toform a spray of small droplets. The droplets are then dried, and theparticles are left in aerosol form. The particles, may then be, forexample, studied or analyzed using downstream analysis devices, e.g.,detectors and apparatus, such as differential mobility particle sizers(DMPS), differential mobility analyzers (DMA), electrometers, andcondensation particle counters (CPC). The charged particles resultingfrom use of the electrospray device may have, for example, a nominaldiameter of about 100 micrometers or less.

As liquid evaporates from the droplets, surface charge density on thedroplets increases until the Rayleigh limit is reached, at which pointthe coulomb repulsive force becomes on the same order as cohesiveforces, such as surface tension. The resulting instability causes theoriginal droplet, sometimes referred to as the parent or primarydroplet, to disintegrate into smaller droplets, thus, the resultingdistribution of droplet size is broad, i.e., nonuniform. One solution tothe problem is to neutralize the droplets and, as such, the particles.

As described in U.S. Pat. No. 5,247,842, a charged neutralizing devicedisposed proximate an electrospray discharge and along an evaporationregion is used to provide the function of reducing an electrical chargeof the droplets as the spray of droplets exits the electrospray deviceto prevent the droplets from disintegrating due to repulsive coulombicforces. For example, the electrospray device produces very highlycharged aerosol particles which typically carry about 80%-95% of aRayleigh limit of charge, on the order of 10-1,000 elementary units ofcharge.

As described in U.S. Pat. No. 5,247,842, a preferred neutralizationprocess includes using a source of ionizing radiation (for example,radioactive polonium emitting alpha particles or a photon ionizationsource), or another source of ions, such as corona discharge. The sourceof ions is positioned proximate the electrospray discharge such that thedroplets encounter the ions virtually immediately upon their formation.Additional sources of ions can be positioned further downstream alongthe evaporation region so that the droplets are further neutralized asthey proceed downstream.

In such a device, the highly unipolarly charged particles, e.g., sprayfrom the electrospray device, are exposed to the ions in theneutralizing device. However, more than 80% of the charged particles arelost within the neutralizer, e.g., to the walls, because of the highelectric mobility of the particles. Further, such particles are lost dueto the use of a high electric field needed for generating dropletsencompassing the charged particles. The charged particles follow thehigh electric field from the point of generation to the walls and manyare lost at the walls. Further, the space charge of the chargedparticles also cause the expansion of the stream of particles resultingin contact and loss to the walls. With such high charged particle loss,the amount of particles reaching the exit for provision to downstreamdevices, such as detection and characterization devices, is undesirablylow.

SUMMARY OF THE INVENTION

In accordance with the present invention, a neutralizing apparatus isdescribed. For example, the neutralizing apparatus may be used with anelectrospray aerosol generator, such as an electrospray device thatgenerates a discharge of droplets and/or particles, or may be used forthe neutralization of any other sources which generate discharges withhigh charge levels. The neutralizing apparatus, according to the presentinvention, provides a flow of ions directed by an electric field. Theflow of ions is counter to the flow of the charged particles/droplets ofthe discharge. As such, the discharge is immersed in the flow ofdirected ions resulting in rapid discharging of the highly chargeddischarge. Particle loss is thereby reduced.

Another method for use in neutralizing a charged discharge according tothe present invention includes providing a neutralizer housing having alongitudinal axis extending between an inlet and an outlet thereof. Acharged discharge is introduced into the inlet of the neutralizerhousing for flow parallel to the inlet of the longitudinal axis from theinlet to the outlet. An alternating electric field is created within thehousing parallel to the longitudinal axis for directing bursts ofnegatively charged ions and positively charged ions alternately towardsthe inlet for use in neutralizing the charged discharge. An apparatusfor carrying out this method is also described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block diagram of a charged particle source inconjunction with a neutralizing apparatus in accordance with the presentinvention.

FIG. 2 is a diagrammatical cross-sectional view of one illustrativeembodiment of a particle neutralizing apparatus in accordance with thepresent invention.

FIGS. 3A, 3B, and 3C are end views and a cross-sectional view,respectively, of the particle neutralizing apparatus of FIG. 2; thecross-sectional view taken through one of the electrodes located towardsthe middle of the particle neutralizing apparatus of FIG. 2.

FIG. 4 is a diagram showing the electric field set up with theelectrodes operational in the particle neutralizing apparatus of FIG. 2.

FIG. 5 is a more detailed cross-sectional illustration of the particleneutralizing apparatus of FIG. 2 in accordance with the presentinvention.

FIG. 6 is a cross-sectional illustration of an alternate embodiment ofthe particle neutralizing apparatus of FIG. 2 in accordance with thepresent invention.

FIG. 7 is a cross-sectional illustration of a portion of yet anotheralternate embodiment of the particle neutralizing apparatus of FIG. 2 inaccordance with the present invention.

FIG. 8 is a block diagram of a system for detecting and characterizingsmall particles using a neutralizing apparatus in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention shall be described generally with reference toFIG. 1. Thereafter, various embodiments shall be described withreference to FIGS. 2-8. One skilled in the art will recognize thatelements from one embodiment of the present invention may be used incombination with elements of one or more other embodiments, and furtherthat the present invention is not limited to the specific illustrativeembodiments described herein but only as described in the accompanyingclaims.

The present invention is directed to apparatus and methods forneutralizing a charged discharge, i.e., a discharge of droplets,particles, or combinations thereof. FIG. 1 shows a neutralizingapparatus 2 having an opening 3 for receiving a highly charged discharge7 from a source 4, e.g., an electrospray device. The highly chargeddischarge 7 flows in a first direction towards the outlet 6 ofneutralizing apparatus 2. Counter to the flow of the highly unipolarlycharged discharge is a unipolar stream of ions 8 having a polarityopposite that of the charged discharge 7. The stream of unipolar ions isdirected towards the inlet 3 of the neutralizing apparatus 2 with use ofan electric field set up in the neutralizing apparatus by generallyrepresentative elements 9 in FIG. 1, e.g., an electrode configuration.The stream of unipolar ions is directed towards the inlet 3 such thatthe ions flow counter to the stream of charged discharge 7. In thismanner, the neutralizing apparatus 2 can be used for neutralizing theunipolarly charged discharge 7. For example, the electrospray device, asdescribed in the Background of the Invention section herein, producesvery highly charged particles which typically carry about 80% to about95% of the Rayleigh limit of charge. Such highly charged particles canbe neutralized according to the present invention. However, one skilledin the art will recognize that lesser charged particles can also beneutralized according to the present invention.

The neutralizing apparatus 2 receives the charged discharge 7 from thecharged particle source 4 through the inlet 3. The charged discharge 7is immediately immersed in the unipolar ions 8 of opposite polaritywhich are directed counter to the flow of the charged discharge 7. Thecharged discharge 7 collides with the unipolar ions 8 resulting in rapiddischarging of the charge on the charged discharge 7. A neutralizedstream is then provided at outlet 6. With such rapid neutralization,particle loss to the walls of a neutralizing apparatus 2 will besubstantially reduced. Therefore, the neutralizing apparatus 2 resultsin a higher output of neutralized particles at outlet 6 of theneutralizing apparatus 2 relative to other conventional neutralizingapparatus.

Although the present invention is advantageous in the neutralization andprovision of submicron particles at outlet 6 of the neutralizingapparatus 2 (i.e., particles having a nominal diameter of less than 1micron), the advantages are even greater for the neutralization andprovision of nanometer particles (i.e., particles having a nominaldiameter of less than about 100 nanometers).

The source 4 for providing the charged discharge 7 to neutralizingapparatus 2 may be any source suitable for providing a chargeddischarge, i.e., particles, droplets, or combinations thereof.Preferably, the source 4 is an electrospray device which provides aunipolarly charged electrospray. The electrospray provided may includeeither droplets encompassing one or more particles and/or chargedparticles themselves. Further herein, the particles and/or dropletsencompassing such particles 7 is referred to as a charged discharge. Acharged discharge is meant to include droplets encompassing particles,particles resulting from the evaporation of the liquid of droplets, astream of a unipolarly charged aerosol, charged particles from anaerosol generator, naturally charged particles such as charged particlesin work environments, charged particles in emissions from engines, etc.Many types of discharges may be charged according to the presentinvention (e.g., polystyrene, silver, biological material, etc.), andthe present invention is not limited in any particular type ofdischarge.

As further described below, the elements 9 for generating an electricfield to direct the unipolar ions toward the inlet 3 of the neutralizingapparatus 2 may take one of many forms. For example, as furtherdescribed below, the elements include electrode rings, a liner ofresistive material, or any other configuration suitable for setting up auniform electric field to direct the unipolar ions toward the inlet 3.Further, the unipolar ions may be provided by various sources such asionizing radiation, for example, radioactive polonium emitting alphaparticles or a photon ionization source, or any other source of ionssuch as a corona discharge. The use of such sources of ions shall befurther described below.

The source 4 for the charged discharge 7 may be, for example, theelectrospray apparatus as described in U.S. Pat. No. 5,247,842. Such anelectrospray device shall be described further below with reference toFIG. 8.

One embodiment of a neutralization apparatus 10 is shown in and shall bedescribed with reference to FIGS. 2-5. The neutralization apparatus 10includes an elongated neutralizer housing 12 having a longitudinal axis11 extending therethrough. The neutralizer housing 12 includes agenerally tubular housing member 14 lying along the longitudinal axis 11which defines at least a portion of a neutralization zone or volume 13.The neutralizer housing 12 further includes a first annular end member16 and a second annular end member 18. The first annular end member 16is connected and sealed to one end of the tubular housing member 14using fastener 17 and seal element 72, and the second annular end member18 is connected to the other end of the tubular housing member 14 withfasteners 19 and seal element 73. One skilled in the art will recognizethat any sort of connection elements may be used in the construction ofthe neutralization apparatus and that the neutralizer housing may beconstructed as a single element or any other number of elements ormembers.

Further, the housing 12 may be constructed of any number ofnonconductive materials such as plexiglas, ceramic, etc. If the housingis formed of a high temperature insulative material, e.g., a hightemperature plastic or a ceramic material, the neutralizing apparatus 10can be baked prior to neutralization processes being performed. Suchbaking, together with the use of high purity gases, allows ions of knownspecies to interact with the charged discharge received by theneutralizing apparatus 10.

An inlet 22 is defined in the first end member 16 of the neutralizerhousing 12 for receiving a charged discharge 20. The charged discharge20, as shown in FIG. 2, is a unipolarly charged (negative) stream ofparticles or droplets encompassing particles flowing towards an outlet26 defined in the second end member 18 of the neutralizer housing 12 forallowing exit of neutralized particles 24. The neutralizer housing 12defines an obstruction-free neutralization zone 13 extending from theinlet 22 to the outlet 26. The flow of the unipolarly charged particlesor discharge 20 has an unobstructed path along the longitudinal axis 11of the neutralizing apparatus 10.

The inlet 22 is defined by an annular inlet member 23 that is concentricwith the first end member 16 about the longitudinal axis 11. A portionof the annular inlet member 23 extends beyond the opening 25 defined inthe first end member 16 to facilitate connection of the apparatus 10 toequipment providing the charged discharge 20. A portion 27 of theannular end member 23 extends to the interior of the first end member 16for defining an annular cavity 34 with a portion of the first end 16 andan annular metal screen 36. The cavity 34 is for receiving air or gas 28through air inlet 30 defined in the first end 16. The portion 27 of theannular inlet member 23 is tapered to promote movement of the air or gas28 in the cavity 34 towards the conductive screen 36 (e.g., a lowporosity screen, sintered metal, perforated metal, or the like) suchthat an annular clean air sheath 52 is provided between the chargeddischarge 20 or other particles flowing along longitudinal axis 11 andthe neutralizer housing 12. In other words, the annular air sheath 52surrounds the charged discharge 20 in the neutralization zone 13preventing particles therein from migrating to the inner surface 47 ofthe neutralizer housing 12. This minimizes loss of particles to theinner surface 47 of the neutralizer housing 12. The clean sheath 52flows substantially parallel to the longitudinal axis 11 adjacent theinner surface 47 from the metal screen 36 to the outlet 26. The cleansheath 52 may be created using any inert gas (e.g., nitrogen, helium,argon), particle free air, or the like. The clean air sheath 52 ispreferably attained by providing a flow of air or gas 28 that is equalto or greater than about 2 times the flow of the charged discharge 20entering the inlet 22. Further, the clean sheath 52 may be a heatedclean sheath to promote evaporation within the neutralization apparatus10.

The outlet 26 is defined by an annular outlet member 27 that isconcentric with the second end member 18 about the longitudinal axis 11.A portion of the annular outlet member 27 extends beyond the opening 35of the first end member 18 to facilitate connection of the apparatus 10to equipment to which the stream 24 of neutralized aerosol particles isprovided, e.g., DMPS, CPC, etc.

The size of the tubular housing member 14 is selected such that lossesof the charged discharge 20 received at inlet 22 to inner surface 47 ofneutralizer housing 12 is minimized. In other words, the size of thetubular housing member 14 is selected so that the charged discharge doesnot reach the wall and result in particle loss. Preferably, the diameterof the tubular housing member 14 and diameter of the inlet 22 isselected such that the mean velocity of the clean sheath isapproximately the same as the mean velocity of the charged discharge.The residence time of particles in the housing is at least in partdependant on the mean velocity of the charged discharge, the meanvelocity of the air sheath, and the cross-section area of the tubularhousing member 14.

The neutralizing apparatus 10 further includes an ion source 38 for usein providing the stream of unipolar ions to the neutralization zone 13.In this particular embodiment, the ion source 38 is a radioactive sourceused to produce bipolar ions positioned in an indented annular slot 63in tubular housing member 14. For example, the radioactive source may bepolonium-210, carbon-14, Kr-85, Ni-63, Am-241, or any other known andsuitable radioactive source for providing bipolar ions. Because of theefficiency of the neutralizing apparatus 10, the source strength of theradioactive source needed to provide for neutralization and preferredoutput of neutralized particles is minimized. The source strength of theradioactive source is preferably less than about 3 millicuries and morepreferably less than or equal to about 0.5 millicuries.

The indented annular slot 63 in tubular housing member 14 is positionedin proximity to the outlet 26 of the neutralizer housing 12 along theinner surface 47 of the tubular housing member 14. A confined uniformelectric field is set up as described further below. The uniformelectric field causes ions of one polarity (positive, as shown in FIG.2) to be pushed towards the inlet 22 of neutralizing apparatus 10 alonglongitudinal axis 11. As such, the directing of unipolar ions towardsinlet 22 provides a flow counter to the flow of charged discharge 20flowing along longitudinal axis 11. The stream of unipolar ions directedtowards the inlet 22 by the electric field in the neutralization zone 13collides with the charged discharge 20. As such, the collisions causethe charged discharge 20 to be neutralized. The neutralization tends tooccur in a neutralization region 15 of the neutralization zone 13 as theunipolar ions approach inlet 22. A bipolar ion region 31 is maintainedproximate the outlet 26 and the radioactive source 38. Therefore, ascharged discharge is received into the neutralization zone 13, it entersneutralization region 15 whereupon collisions with unipolar ions ofopposite charge neutralize the charged discharge 20. As the neutralizeddischarge proceeds through neutralization zone 13, further evaporationoccurs resulting in neutralized particles flowing into bipolar ionregion 31. As the bipolar ion region 31 is maintained with both negativeand positive ions, the neutralized particles are not overcharged to onepolarity or the other prior to exiting through outlet 26.

Generally, the confined uniform electric field, which is substantiallyparallel to longitudinal axis 11 used for directing the unipolar ionstream towards inlet 22, is generated using a drifting tube electrodeconfiguration 44. The confined uniform electric field 70 is generallyshown in FIG. 4 with the field being set up by utilizing annularinsulating ring electrodes 46 distributed along longitudinal axis 11.The ring electrodes are preferably placed equal distances apart. Anysuitable number of ring electrodes may be used to create the field,preferably 5 or more ring electrodes.

Voltages, ramped in level from inlet to outlet, are applied to theelectrodes 46 by one or more power sources generally represented byreference arrow 40 pointing out the various voltages being applied. Inother words, as shown in FIG. 1, the ring electrode 46 proximate theoutlet has an applied voltage of -x volts, and the ring electrodeproximate the inlet 22 has an applied voltage of -y volts. The ringelectrodes 46 between the inlet and outlet have an applied voltagesomewhere between -x volts and -y volts in a ramped manner. With thevoltage proximate screen 36 being more negative than proximate theoutlet 26, positive ions are directed towards inlet 22. With the DCvoltages applied, the drifting tube electrode configuration 44 allows aconfined uniform electric field 70 to be set up parallel to the flow ofthe charged discharge 20 entering inlet 22 and is suitable for directinga stream of unipolar ions of opposite polarity towards the chargeddischarge 20 or inlet 22.

One skilled in the art will recognize that the negative or positivenature of the voltages applied will cause the unipolar ions in theneutralization region 15 to be either positive or negative when abipolar source is used. The reversal of the voltages applied, includingthe reversal of ramped nature of such voltages throughout thedescription herein, are contemplated within the scope of the presentinvention to achieve both positive and negative ion neutralization ofoppositely charged discharges. The particular voltages used forillustration herein are not to be construed as being unduly limiting tothe present invention, as the present invention is limited only inaccordance with the accompanying claims.

When the charged discharge 20 collides with the stream of unipolar ionsflowing counter thereto in region 15, the charged discharge 20 israpidly discharged and the neutralized particles move towards and exitthrough outlet 26 under the assistance of the air sheath 52.

The confined uniform electric field 70 is defined as a field which issubstantially uniform in a core region 99 about the longitudinal axis 11in the neutralization zone 13 with no uncontrolled fringing of theelectric field from the core region 99 to the neutralizer housing 12.The confined field 70 is generally parallel to the longitudinal axis andin a direction towards the inlet 22. Controlled fringing between thering electrodes 46 is shown by reference numeral 77 in FIG. 4. Thisfringing does not run from the core region 99 to the neutralizer housingand is controlled or confined by use of the air sheath 52. For example,the air sheath 52, the design or configuration of the neutralizerhousing 12, and/or the electrode configuration, can be optimized toencompass the maximum possible field fringing of the electrodes 46within the air sheath, e.g., the air sheath width can be set 2 times themaximum distance the fringing extends into the neutralization region 13.Such fringing does not exist with the continuous resistor layerconfiguration described further below with reference to FIG. 6.

FIGS. 3A-3C are end views and a cross-sectional view, respectively, ofthe neutralizing apparatus 10 of FIG. 1. The cross-sectional view ofFIG. 3C is taken through one of the ring electrodes 46 located towardsthe middle of the neutralizing apparatus 10.

One particular embodiment of a neutralizing apparatus according to thepresent invention is shown in FIG. 5. This embodiment includes moredetail relative to the neutralizing apparatus 10, shown in FIG. 2, anduses the same reference numerals as used therein to designate same orsimilar elements. The neutralizing apparatus 10, as shown in FIG. 5,includes a ring electrode configuration 44 such as described withreference to FIG. 2. In this more detailed illustration of the apparatusof FIG. 2, the voltages are applied to the various electrodes using asingle power source 102. The voltage ramp of the ring electrodes 46 isset up by placing resistors 110 between adjacent electrode rings 46. Theelectrode 46 proximate outlet 26 is connected to ground.

In alternate configurations with respect to all the illustrativeembodiments described herein, the power source for applying voltages tothe various electrodes may be AC sources. When AC voltages are applied,an oscillating electric field is generated. For example, with such anoscillating electric field, bursts of positively charged ions andnegatively charged ions are alternately directed into neutralizationregion 15. In this manner, charged discharges other than unipolarlycharged discharges can be neutralized effectively. The power source 102is meant to represent either the application of DC or AC voltages.

The configured electric field 70 (FIG. 4) can also be set up using thealternate drifting tube electrode configuration shown in theillustration of FIG. 6. As shown therein, the ring electrodes arereplaced by thick film resistor 202 of uniform thickness on a portion ofthe nonconducting inner surface 203 of a neutralizer housing 206. Theresistor layer 202 extends from a first end 204 to a second end 205which is connected to the metal screen 210. When a voltage is applied tothe second end 205 of the resistor layer 202, the voltage ramps (e.g.,becomes less negative) along the axial direction towards the second end204, which is adjacent electrode 209 that is electrically grounded. Thiselectrode configuration also provides a confined uniform electric fieldin region 211 parallel to the flow of charged discharge 20 and suitablefor directing a unipolar stream of ions counter to the charged dischargeflow.

In another alternate illustration of a portion of a neutralizingapparatus 300, shown in FIG. 7, the neutralizer housing 301 defines aneutralization zone 303. Corona discharge rings or discs 302 having asharpened edge are distributed along the housing 301 for providing freeelectrons and/or ions to the neutralization zone 303. The free electronsand/or ions are then used for neutralizing the stream of chargedparticles 310 flowing along longitudinal axis 311 of the apparatus 300.The corona discharge rings or discs have a high voltage applied thereto(e.g., negative or positive). A plurality of perforated or porous metalplates or screens 304 are distributed along the housing 301 between thesharpened edges of the corona discharge discs and the neutralizing zone303. The perforated plates 304 allow the corona discharge to be formedabout the sharpened edge with free electrons and/or ions being drawninto the neutralization zone 303. The perforated plates 304 having rampvoltages applied thereto create the confined electric field, asdescribed previously with respect to other embodiments herein, inaddition to providing a lower reference voltage relative to the highvoltage applied to the corona discharge elements for creation of thecorona discharge. However, since this embodiment provides for a highconcentration of free electrons along the entire neutralization zone303, directing of the free electrons towards the inlet of the apparatus300 may not be necessary. As this particular embodiment does not providefor a bipolar ion region to prevent overcharging of particles that havebeen neutralized prior to exit of neutralizing apparatus 300, theconcentration of ions in the neutralization zone 303 must be controlled.In other words, the concentration of free electrons and/or ions in theneutralization zone 303 must be kept at a certain level such thatparticles exiting the neutralization apparatus 300 are not charged afterbeing neutralized prior to exit from the apparatus 300. Alternatively,to prevent overcharging, a bipolar ion region 313 may be positionedadjacent the exit to avoid overcharging from the neutralizing apparatus300. For example, a radioactive source may be used to provide bipolarions in this region 313.

FIG. 8 shows a system for detecting and characterizing particles using aneutralization apparatus 2 and having an electric field for directing astream of unipolar ions toward the inlet of the neutralization apparatus2, as described previously herein. The system 400, other than theneutralizing apparatus 2, is described in further detail in U.S. Pat.No. 5,247,842, and shall not be described in detail herein. It will berecognized by one skilled in the art that the various components orelements described for providing an electrospray to neutralizationapparatus 2, including heaters for providing evaporation and alsoelements for receiving the neutralized particles, may be or take manydifferent forms. However, as shown in FIG. 8, the system 400 includesparticles as part of a liquid sample or solution held in a container424. A syringe pump 422 loaded with liquid from the container suppliesthe liquid to an electrospray chamber 404. Another input to electrospraychamber 404 is a steady rate supply of filtered gas, typically air. Moreparticularly, air under pressure in a container 414 is provided througha valve 412 to a filter 410, and then via controlling orifice 408 to theelectrospray chamber 404. Such gas may also be heated. A high voltagesource 420 is electrically connected to a capillary needle 406 of theelectrospray device 404 while portions of the electrospray device areisolated from the needle and are connected to ground. To provide theelectrospray, a high potential difference between the needle 406 and anisolated portion is provided. It will also be recognized that air from asupply 434 may be directed through a valve 432, a filter 430, and acontrolling orifice 428 to a heater 426 to be provided to theneutralizing apparatus 2 to maintain the temperature within theneutralizing apparatus 2. This promotes evaporation of droplets of theliquid sample.

Further, the system 400 includes an output from the neutralizingapparatus 2 which is provided to a differential mobility particle sizer(DMPS) 402 and/or a charge spectrometer 403. The DMPS consists of anelectrostatic classifier 452, a condensation particle counter 454, and asuitable microcomputer 450. Such components for use in sizing andclassifying particles are available from TSI Incorporated. The chargespectrometer 403 is used to provide a charge distribution of the streamof particles exiting the neutralizing apparatus 2.

Almost immediately, as droplets emerge from capillary needle 406 ofelectrospray device 404, the solution droplets begin to shrink due toevaporation of the volatile solvent of the sample solution. If each ofthe droplets were to retain its electrical charge, the surface chargedensity would increase as the droplet size is reduced. Eventually, thecoulombic forces would overcome cohesive forces, such as surfacetension, causing each droplet to disintegrate into a plurality ofsmaller droplets. Coulomb disintegration, occurring generally throughoutthe aerosol, would destroy the size uniformity of the droplets. With theuse of the neutralizing apparatus 2, in accordance with the presentinvention, the unipolar stream of ions encounter the droplets from theelectrospray device 404 and reduce their electrical charge at the inlet,neutralizing such droplets. This minimizes the potential for dropletdisintegration due to coulombic forces as the droplet proceeds andevaporates in the neutralizing apparatus 2. Further, by neutralizing thecharged spray immediately after discharge into the neutralizingapparatus, particle loss to the walls of the neutralizing apparatus 2are reduced.

All patents and references disclosed herein are incorporated byreference in their entirety, as if individually incorporated. Further,although the present invention has been described with particularreference to various embodiments thereof, variations and modificationsof the present invention can be made within the contemplated scope ofthe following claims, as is readily known to one skilled in the art.

What is claimed is:
 1. A method for use in neutralizing a chargeddischarge, the method comprising the steps of:providing a neutralizerhousing having a longitudinal axis extending between an inlet and anoutlet of the neutralizer housing; introducing a charged discharge intothe inlet of the neutralizer housing for flow parallel to the inlet ofthe longitudinal axis from the inlet to the outlet; and creating analternating electric field within the housing parallel to thelongitudinal axis for directing bursts of negatively charged ions andpositively charged ions alternately towards the inlet for use inneutralizing the charged discharge.
 2. The method according to claim 1,wherein the step of creating the alternating electric field within thehousing includes:positioning a plurality of ring electrodes along thelongitudinal axis; and applying a plurality of AC voltages ramped inlevel from a first ring electrode to a last ring electrode of theplurality of ring electrodes lying along the longitudinal axis.
 3. Themethod according to claim 1, wherein the neutralizer housing includes anonconducting tubular portion having an inner surface, and furtherwherein the step of creating the alternating electric field within thehousing includes:providing a layer of resistive material having a firstend and a second end, the layer of resistive material extending along alength of the inner surface of the nonconducting tubular portion;applying an AC voltage to the first end; and grounding the second end.4. The method according to claim 1, wherein the method further includescreating a clean sheath between the charged discharge and theneutralizer housing.
 5. A neutralizing apparatus, the apparatuscomprising:an elongated neutralizer housing having a longitudinal axisextending between an inlet and an outlet defined therein, the inlet forreceiving the charged discharge; and an electrode configuration operableto create an alternating uniform electric field within the neutralizerhousing parallel to the longitudinal axis for directing bursts ofnegatively charged ions and positively charged ions alternately towardsthe inlet for use in neutralizing the charged discharge.
 6. Theapparatus according to claim 5 wherein the apparatus further includesmeans for creating a clean sheath between the charged discharge and theneutralizer housing.
 7. The apparatus according to claim 5, wherein theelectrode configuration includes:a plurality of ring electrodes locatedalong the longitudinal axis; and one or more AC sources for use inapplying a plurality of voltages ramped in level from a first ringelectrode to a last ring electrode of the plurality of ring electrodeslying along the longitudinal axis.
 8. The apparatus according to claim5, wherein the elongated neutralizer housing includes a nonconductingtube portion having an inner surface, wherein the electrodeconfiguration includes a layer of resistive material having a first endand a second end located along a length of the inner surface of thenonconducting tube portion, and further wherein an AC power source isconnected for use in applying an alternating voltage to the first endwith the second end grounded.